1. Field of the Invention
The present invention relates to an image pick-up tube and, specifically, to a structure of a target portion of an image pick-up tube of a photoconductive type for visible light or for x-rays. More particularly, the present invention relates to improvement of the target portion suitable for the image pick-up tube used with a increased target voltage.
2. Description of the Related Art
Generally, a photoconductive type image pick-up tube or an x-ray image pick-up tube (to be called an image pick-up tube as a general term hereinafter) includes a target portion for converting an incident light or x-ray image into a charge pattern to accumulate a charge, and an electron beam scanning section for reading the charge pattern to obtain a current signal. Immediately after the target portion is scanned by an electron beam, the image pick-up tube operates to balance a surface potential on the scanned side of the target portion with a cathode potential. Such an image pick-up tube is described in, for example, "Satuzou Kougaku (imaging engineering)", pp. 109-116, by Ninomiya et al., published by Corona-sha, 1975, IEEE Electron Device Letters, 1987, EDL-8, No. 9, pp 392-394, or Preliminary Transactions for National Congress of the Institute of Television Engineers of Japan, 1982, pp. 81-82, by Kawamura, et al.
With such an image pick-up tube, if secondary electrons are easily emitted from a surface of a target portion on a side scanned by an electron beam, the normal operation mentioned above of the image pick-up tube cannot be achieved. In order to decrease a secondary electron emission yield on the scanned side surface, a method is disclosed, in which an electron beam landing layer composed of porous Sb.sub.2 S.sub.3 is deposited on the scanned side surface of the target portion in inert gas by evaporation (JP-B-52-40809). In addition, in order to suppress a false signal generated when excess electrons are reflected by an electrode in the tube during scanning an electron beam and come into the target portion again so as to obtain an output signal of a high S/N ratio, some methods are disclosed; in one method, a new electrode is provided on an area of the scanned side surface of the target portion except for an area scanned by an electron beam (JP-A-61-131349) and in another method, a transparent conductive film of the target on the light or x-ray incident side is separated into two portions in correspondence with a portion scanned by an electron beam and the other portion and the two portions are connected to individual power supplies to control an operation of the tube (JP-A-63-72037).
With the image pick-up tube according to the above conventional methods, it is necessary to make a photoconductive layer of target portion thick in order to increase its sensitivity and decrease capacitive after-image. In addition, it is necessary to make a voltage between a target electrode and a cathode electrode of the image pick-up tube (to be referred as a target voltage hereinafter) higher in order to cause avalanche multiplication in the photoconductive layer so as to realize higher sensitivity. However, if such an image pick-up tube is used with the higher target voltage, a bad phenomena such as a phenomenon that image distortion or shading appears in a reproduced image on a monitor; a phenomenon that an abnormal pattern varying in a ripple manner is generated in the periphery of the reproduced image (to be referred to as a water fall phenomenon), and a phenomenon that the polarity of a portion of an output signal from the tube corresponding to the periphery of the reproduced image is inverted (to be referred to as a signal inversion phenomenon). Therefore, there is, in the conventional image pick-up tube, a problem in which it is difficult to stably obtain a better reproduced image.